Prior art subsea test valve assemblies are generally adapted to be located in a subsea blowout preventer stack, and enable removal of tubing and related equipment from a subsea petroleum recovery well. Such assemblies may include an upper subassembly releasably latched to a lower subassembly to permit the drill pipe or tubing string above the test tree to be released from the lower subassembly and components connected thereto, so as to temporarily abandon the well during incumbent weather or other unsafe or undesirable conditions. One or more valves within the lower subassembly, normally open during production of petroleum fluids, are closed prior to the unlatching of the upper subassembly. These valves may be reopened to resume production operations after the drilling vessel returns to the site and the upper subassembly is reconnected to the lower subassembly.
U.S. Pat. No. 3,457,991 discloses an automatic safety valve assembly positioned adjacent the blowout preventers. Fluid pressure from the surface opens a pair of ball valve elements from their biased closed position.
U.S. Pat. No. 3,870,101 discloses a subsea production test valve assembly including one or more pressure activated ball valves, and a latch mechanism for releasably securing the upper subassembly to the lower subassembly containing one or more valves. The pistons controlling the valves are pressure balanced, and the valves are adapted to permit reverse flow around them when in the closed condition. The piston for manipulating the lower valve to the closed position drives a ball, which cuts a wireline extending through the valve which would otherwise hold the valve partially open. The latching mechanism may be hydraulically or mechanically controlled once the ball valves are in the closed position. In order to mechanically release the latch, the tubing string is rotated, unthreading latch fingers within the lower torque sub.
U.S. Pat. No. 4,320,804 discloses another valve assembly suitable for locating in a subsea blowout preventer stack. One ball valve element may be manipulated from a closed to an opened position only subsequent to another ball valve element being manipulated to the fully open position, thereby assuring more efficient cutting of the wireline through the ball valve elements. The latching apparatus comprises first and second rocker sections selectively controlled by a collet shiftable by a piston and a spring. Again, the latch may be hydraulically or mechanically manipulated, which is accomplished in the latter instance by rotating the tubing string until latch fingers are moved away from locking engagement. The stroke of the latching mechanism is approximately 3.5 inches, which inherently increases the length of the assembly and accordingly excludes its utility for certain applications.